EP0072573A2

EP0072573A2 - Process for producing pitch for use as raw material for carbon fibers and carbon fibers produced from the pitch

Info

Publication number: EP0072573A2
Application number: EP82107538A
Authority: EP
Inventors: Kunihiko Moriya; Kazuhito Tate; Goro Muroga; Kazuhiro Yanagida
Original assignee: Mitsubishi Oil Co Ltd
Current assignee: Eneos Corp
Priority date: 1981-08-18
Filing date: 1982-08-18
Publication date: 1983-02-23
Also published as: US4462894A; DE3272453D1; EP0072573B1; JPS61876B2; JPS5829885A; EP0072573A3

Abstract

process for producing a pitch is disclosed. The process involves distilling a heavy petroleum oil under reduced pressure to obtain a distillate, the distillate is hydrogenated to obtain a hydrogenated oil which is subjected to catalytic cracking. The cracked oil is subjected to distillation to obtain a high boiling point fraction having a boiling point of more than 300°C. The high boiling point fraction is subjected to thermal modification in order to obtain the pitch. The pitch can be utilized in order to produce carbon fibers of high quality. By utilizing the process, a greater variety of starting materials can be utilized in order to produce pitch which can be utilized to produce high quality carbon fibers.

Description

The present invention relates to a process for producing pitch (which is a raw material for producing carbon fibers) using a petroleum heavy residual oil, and carbon fibers produced from the pitch.
In pitches which are used as a raw material for producing carbon fibers having excellent strength and excellent modulus of elasticity, optical anisotropy can be observed by a polarizing microscope. More specifically, such pitches are believed to contain a mesophase as described in U.S. Patent 3,974,264. Further, it has recently been disclosed in Japanese Patent Application (OPI) 160427/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") that carbon fibers having a high modulus of elasticity can be produced with a pitch containing a neo-mesophase. By heating such pitches for a short time optical anisotropy is observed in them. Further, pitches used as a raw material for carbon fibers need not possess only optical anisotropy but must also be capable of being stably spun. However, it is not easy to produce pitches having both properties. In order to produce carbon fibers having excellent strength and excellent modulus of elasticity, it is not always possible to use any material as the raw material for making pitches. Materials having specified properties have been required.
It should be noted that in many published patents, for example, as described in U.S. Patents 3,976,729 and 4,026,788, the raw material is not specified in the patent claims. Furthermore, such patents indicate that pitches used as a raw material for carbon fibers can be produced only by carrying out thermal modification of a wide variety of raw materials. However, according to the detailed descriptions and examples in such patents, the desired pitches can only be produced by using specified raw materials.
For example, U.S. Patent 4,115,527 discloses that substances such as chrysene, etc. or tarry materials obtained as by-products in high.temperature cracking of petroleum crude oil are suitable for producing the pitch, i.e., a carbon fiber precursor, but conventional petroleum asphalts and coal tar pitches are not suitable. Further, U.S. Patent 3,974,264 discloses that an aromatic base carbonaceous pitch having a carbon content of about 92 to about 96% by weight and a hydrogen content of about 4 to about 8% by weight is generally suitable for controlling a mesophase pitch. It has been described that elements other than carbon and hydrogen, such as oxygen, sulfur and nitrogen, should not be present in an amount of more than about 4% by weight, because they are not suitable. Further, Example 1 of the same patent publication discloses that the precursor pitch used has properties comprising a density of 1.23 g/cc, a softening point of 120°C, a quinoline insoluble content of 0.83% by weight, a carbon content of 93.0%, a hydrogen content of 5.6%, a sulfur content of 1.1% and an ash content of 0.044%. Even if a density of 1.23 g/cc in these properties is maintained, it should be noted that it is difficult to obtain conventional petroleum heavy oil having such a high density. Examples as described in the other U.S. Patents 3,976,729, 4,026,788 and 4,005,183 also disclose that the pitch is produced with a specified raw material.
The properties of heavy petroleum oils depend essentially upon the properties of crude oils from which they were produced and the process for producing the heavy oil. However, generally, it is rare that heavy oils having the suitable properties described in the above-described Examples are produced, and, in many cases, they can not be obtained. Accordingly, in order to produce carbon fibers industrially in a stabilized state, which have excellent strength and excellent modulus of elasticity with petroleum heavy oils, it is necessary to develop a process for producing a pitch wherein the finally resulting pitch has properties which are always within a specified range even if the properties of the raw material for the pitch vary.
An object of this invention is to provide a process for producing a pitch useful as raw material for producing carbon fibers having an excellent strength and a high modulus of elasticity.
In accordance with one aspect of the invention, this object is achieved by a process for producing a pitch used as a raw material for carbon fibers characterised by carrying out hydrogenation treatment of a reduced pressure distillate oil prepared by reduced pressure distillation of a petroleum heavy residual oil, carrying out catalytic cracking of the resulting hydrogenated oil, distilling the resulting cracked oil to produce a high boiling point fraction having a boiling point of more than 300°C, and carrying out thermal modification thereof.
According to another aspect of the invention, a pitch produced according to the method may be used for the manufacture of carbon fibers.
According to a further aspect of the invention, there is provided carbon fibers produced from a pitch produced in accordance with the method.
For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely for purposes of illustration, to a number of non-limiting examples of methods in accordance with the invention.
Examples of petroleum heavy residual oils which may be used in the present invention include atmospheric pressure distillation residual oils and heavy residual oils from a thermal cracking process such as visbreak- ing, etc. The petroleum heavy residual oils having a boiling point of more than 300°C is preferred. The atmospheric pressure distillation residual oils are most commonly used.
The above-described petroleum heavy residual oils can be processed by a reduced pressure distillation apparatus to obtain a distillate fraction. 95% or more of the distillate fraction has a boiling point of 300 - 550°C (atmospheric pressure). The resulting heavy fraction is subjected to hydrogenation treatment in the presence of a conventional hydrogenating catalyst (e.g., a catalyst containing the sulfides or oxides of such combination of metals as nickel-molybdenum, cobalt-molybdenum, etc.) at a temperature of 300 - 410°C, a pressure of 40 - 150 kg/cm²G, a liquid space velocity of 0.5 - 3.0 per hour, and a ratio of hydrogen/oil of 260 - 2,000 Nm³/Kl. By carrying out this treatment impurities such as sulfur, nitrogen or metals are removed from the reduced pressure distillate oil. The resulting hydrogenated oil preferably has a sulfur content of not more than 0.4% by weight.
When producing carbon fibers having a high modulus of elasticity, it is necessary to remove sulfur in the pitch, because a high modulus of elasticity cannot be obtained if the sulfur content of the pitch is large. It is preferred to remove the sulfur prior to the final step, because it is difficult to remove sulfur from the pitch in the final step. It is also necessary to remove metals which form ash by carbonization. Such metals can cause deterioration of the strength or modulus of elasticity of carbon fibers.
The above-described hydrogenated oil is subjected to a catalytic cracking reaction in the presence of a catalytic cracking catalyst comprising amorphous silica-alumina, silica-magnesia or zeolite catalysts. The catalytic cracking reaction is carried out at a temperature of 470 - 540°C, a pressure of 0.5 - 5.0 kg/cm²G and a ratio of catalyst/oil of 5 - 15 parts by weight. A high boiling point fraction having a boiling point of more than 300°C is obtained by distillation of the resulting cracking oil.
The resulting high boiling point fraction is subjected to thermal modification at a temperature of 390 - 430°C for 1 - 30 hours, by which a pitch which can be used as a raw material for making carbon fibers having a high modulus of elasticity can be produced: In the residual heavy fraction after the catalytic cracking reaction, the difference in properties due to any disparity in the raw material becomes smaller due to the effects of the catalytic reaction together with the above-described hydrogenation treatment. Further, the residual heavy fraction develops a chemical composition comprising a large amount of aromatic compounds.
The actual conditions required to obtain the best results in the above-described series of steps depend on the properties of the petroleum heavy residual oil which is used as a starting material as well as the properties of the pitch which will be used as a raw material for making carbon fibers as the final product. By carrying out a series of these steps any difference due to properties of the starting material becomes smaller. Therefore, by carrying out these steps, it is possible to keep the properties of the pitch which is used as a raw material for making carbon fibers within a specified range. Since the properties of the petroleum heavy residual oil (used as the starting material) are fairly different from others because of the crude oil, it is generally difficult to produce pitch (which can be successfully used to make carbon fibers having high strength and high modulus of elasticity and specified properties) by only carrying out the thermal modification of such petroleum heavy oil at 380°C to 450°C.
However, a pitch which can be used as a raw material for carbon fibers having high modulus of elasticity can be produced industrially and stably with various kinds of petroleum heavy residual oils. The pitch is produced by carrying out a series of processings comprising reduced pressure distillation ) hydrogenation treatment e catalytic cracking ) distillation 7 thermal modification. By carrying out these steps it is possible to use a raw material which could not be used for producing a pitch for carbon fibers in accordance with prior processes.
In the following, the present invention is illustrated in greater detail by further specific examples. However, this invention is not limited to these examples.

EXAMPLE 1

An atmospheric pressure distillation residual oil of Middle East crude oil (A) was subjected to reduced pressure distillation to obtain a fraction having a boiling point of 300 - 550°C (an atmospheric pressure). The resulting reduced pressure distillation fraction was subjected to hydrogenation treatment in the presence of a cobalt-molybdenum catalyst. The hydrogenation was carried out at a temperature of 370°C, a pressure of 60 kg/cm²G, a liquid space velocity of 1.9 per hour and a ratio of hydrogen to oil of 360 Nm³Kl. The hydrogenated oil was subjected to a catalytic cracking reaction using a zeolite catalyst. The cracking was carried out at a temperature of 500°C, a pressure of 1.5 kg/cm 2 G and a catalyst/oil ratio of 9 parts by weight. The residual heavy oil obtained from the catalytic cracking reaction was distilled to obtain a high boiling point fraction having a boiling point of more than 300°C. The high boiling point fraction was subjected to thermal modification at a temperature of 410°C for 20 hours to obtain a pitch which could be used as a raw material for making carbon fibers.
The properties of the atmospheric distillation residual oil of Middle East crude oil (A) used as a raw material, and the properties of the oil after hydrogenation treatment, as well as the properties of the high boiling point fraction after catalytic cracking and the properties of the resulting pitch are shown in the appended Table 1.
Carbon fibers were obtained by melt spinning the above-described pitch at 360°C infusiblizing at 260°C in air and carbonizing at 1,000°C. The resulting carbon fibers had a tensile strength of 11 tons/cm² and a modulus of elasticity of 1,300 tons/cm². When carbonized fibers prepared by carbonizing at 1,000°C were additionally graphitized at 1,900°C, the resulting carbon fibers had a tensile strength of 15 tons/cm² and a modulus of elasticity of 2,300 tons/cm .

EXAMPLE 2

An atmospheric pressure distillation residual oil of Middle East crude oil (B) was subjected to reduced pressure distillation to obtain a fraction having a boiling point of 300 - 550°C (at atmospheric pressure). The resulting reduced pressure distillation fraction was subjected to hydrogenation treatment in the presence of a cobalt-molybdenum catalyst. The hydrogenation was carried out at a temperature of 380°C, a pressure of 60 kg/cm², a liquid space velocity of 1.8 per hour and a ratio of hydrogen per oil of 400 Nm³/Kl. The hydrogenated oil was subjected to a catalytic cracking reaction with a zeolite catalyst. The cracking was carried out at a temperature of 500°C and a pressure of 1.5 Kg/cm² and a catalyst/oil ratio of 9 parts by weight. The residual heavy oil obtained from the catalytic cracking reaction was distilled to obtain a high boiling point fraction having a boiling point of more than 300°C. The high boiling point fraction was subjected to heat treatment at a temperature of 420°C for 10 hours to obtain a pitch which could be used as a raw material for making carbon fibers.
The properties of the atmospheric pressure distillation residual oil of Middle East crude oil (B) used as the raw material, and the properties of the oil after hydrogenation treatment, as well as the properties of the high boiling point fraction after catalytic cracking treatment and properties of the pitch are shown in the appended Table 1.

COMPARATIVE EXAMPLE 1

An atmospheric pressure distillation residual oil of Middle East crude oil (A) was subjected to thermal modification at a temperature of 410°C for 18 hours. The properties of the atmospheric pressure distillation residual oil of Middle East crude oil (A) used as a raw material and the properties of the pitch are shown in the appended Table 1.
Fibers were obtained by melt spinning the pitch at 350°C, infusiblizing in the air and carbonizing at 1,000°C. The fibers obtained had a tensile strength of 1.9 tons/cm² and a modulus of elasticity of 140 tons/cm .

COMPARATIVE EXAMPLE 2

An atmospheric pressure distillation residual oil of Middle East crude oil (A) was subjected to reduced pressure distillation to obtain a fraction having a boiling point in the range of 300 - 550°C. The resulting reduced pressure distillation fraction was subjected to thermal modification at a temperature of 410°C for 20 hours. The yield of the pitch obtained after the heat treatment was low and it was not possible to obtain the pitch in an amount necessary to examine its properties.

COMPARATIVE EXAMPLE 3

An atmospheric pressure distillation residual oil of Middle East crude oil (A) was subjected to reduced pressure distillation to obtain a fraction having a boiling point in the range of 300 - 550°C (at atmospheric pressure). The resulting reduced pressure distillation fraction was subjected to a catalytic cracking reaction using a zeolite catalyst. The cracking was carried out at a temperature of 500°C, a pressure of 1.5 kg/cm²G and a catalyst/oil ratio of 9 parts by weight without the hydrogenation treatment. The residual heavy oil obtained by the catalytic cracking reaction was distilled to obtain a high boiling point fraction having a boiling point of more than 300°C. The high boiling point fraction was subjected to thermal modification at a temperature of 410°C for 20 hours to obtain a pitch.
The properties of the atmospheric distillation residual oil of Middle East crude oil (A) used as a raw material, the properties of the high boiling point fraction after catalytic cracking as well as the properties of the resulting pitch are shown in the appended Table 1.
The pitch obtained was subjected to melt spinning at about 365°C. However, the fiber obtained by the melt spinning broke frequently as compared with the pitch used as a raw material of Example 1. Accordingly, the melt spinning was very difficult to carry out. Further, the melt-spun fiber was infusiblized at 260°C in the air and then carbonized at 1,000°C. The resulting product had a tensile strength of 9 tons/cm² and a modulus of elasticity of 1,010 ton/cm². When the carbonized fibers prepared by carbonizing at 1,000°C were additionally graphitized at 1,900°C, they had a tensile strength of 10 tons/cm² and a modulus of elasticity of 1,610 ton/cm².
Thus, there has been described a process for producing a pitch which can be used for producing carbon fibers having excellent properties industrially in a stabilized state, and making use of an easily available petroleum heavy residual oil.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof as defined in the following claims.

Claims

1. A process for producing a pitch for using as a raw material for carbon fibers, characterized by:

distilling a petroleum heavy residual oil under reduced pressure to produce a reduced pressure distillate oil;

hydrogenating the reduced pressure distillate oil to obtain a hydrogenated oil;

carrying out catalytic cracking of the hydrogenated oil to obtain a cracked oil;

distilling the resulting cracked oil to produce a high boiling point fraction having a boiling point of more than 300°C; and

thermally modifying the high boiling point fraction to obtain the pitch.

2. A process for producing a pitch as claimed in claim 1 wherein the petroleum heavy residual oil is an atmospheric pressure distillation residual oil of crude oil.

3. A process for producing a pitch as claimed in claim 1 wherein, when the petroleum heavy residual oil is distilled under reduced pressure, 95% or more of the distillate fraction has a boiling point of 300 to 550°C at atmospheric pressure.

4. A process for producing a pitch as claimed in any of claims 1, 2 or 3 wherein the hydrogenation treatment of the reduced pressure distillate oil is carried out in the presence of a hydrogenating catalyst at a temperature of 300 to 4l0°C, a pressure of 40 - 150 kg/cm 2 G, a liquid space velocity of 0.5 - 3.0 per. hour and a ratio of hydrogen/oil of 260 - 2,000 _Nm³/_Kl to reduce a sulfur content to less than 0.4% by weight.

5. A process for producing a pitch as claimed in any of claims 1, 2 or 3 wherein the hydrogenated oil has a sulphur content of 0.4% by weight or less.

6. A process for producing a pitch as claimed in any of claims 1, 2 or 3 wherein the cracking is carried out in the presence of a catalytic cracking catalyst under a condition comprising a temperature of 470 - 540°C, a pressure of 0.5 - 5.0 kg/cm²G and a ratio of catalyst/oil of 5 - 15 parts by weight.

7. A process for producing a pitch as claimed in any of claims 1, 2 or 3 wherein the thermal modification is carried out at a temperature of 390 - 430°C for 1 to 30 hours.

8. A process for producing a pitch as claimed in any of claims 1, 2 or 3 wherein the catalyst utilized within the catalytic cracking is a catalyst selected from the group consisting of amorphous silica-alumina, silica-magnesia or zeolite catalysts.

9. Use of a pitch produced according to any one of the preceding claims for the manufacture of carbon fibers.

10. Carbon fibers produced from pitch produced in accordance with any one of claims 1 to 8.